In the summer, attendance at our church falls noticeably as people go on vacation and spend weekend mornings doing other seasonal things, like birdwatching. After the service on a recent Sunday, we all headed out of the sanctuary, toward the atrium. Normally, this is a time when it’s really difficult for me to talk with anyone because of the reverberant nature of our building. It’s an architectural masterpiece and wonderful for music—and an acoustical nightmare, at least for speech comprehension.

To be fair, our church is not the only one with a large, open worship space where sound bounces around for what can seem like…. forever. It’s actually becoming more common; when churches get bigger, sound challenges follow. As the authors of a research paper on the topic point out, “We are witnessing a paradigm shift from small church enclosures to very large church auditoriums. Most of these auditoriums fall short of providing good sound quality and… sooner or later it becomes a very serious problem because such buildings are places for communication to an audience.…”

So, I’ve gotten used to the reverberation, and just try to avoid conversation until we’re out of the sanctuary. That summer day, however, as I worked my way toward the exit, I noticed that the noise level was significantly louder than usual. “That’s weird,” I thought, "fewer people, but more noise?” I checked with a couple of friends, and they had also noticed that the noise level seemed much higher than usual. So it wasn’t just me.

When I got home, I told my (physicist) husband about it, and he asked me how many people were at the service. I said, "Way fewer, less than half the usual number…probably vacations.” He replied, "Oh, that’s probably why it was noisier. People absorb sound.” But at such a noticeable level?

Ask an Acoustician

In search of a second opinion, I contacted Rich Peppin, the president of Engineers for Change, a nonprofit acoustics and vibrations consulting firm. Rich had helped me with a Hearing Health article, “Caution: Noise at Work,” so I knew he’d have the answer. I posited our working hypothesis in my email to him: that a reverberant space would be noticeably noisier if there are fewer people in it.

Rich replied: “Yes. Because people absorb sound and hence reduce reflections. We can calculate the reduction of reverberation if we know before and after numbers of people.” Now, we’re getting somewhere.

The calculations Rich was talking about are based, in part, on how much sound humans absorb. In addition to the sound absorption by human bodies, there are other variables that impact reverberation, such as: what the people are wearing, whether they are sitting or standing, whether there are padded seats in the room, and the size and shape of the room.

In my church example, however, most of the major variables were unchanged between winter and summer: lightly padded seats with metal frames; hard floor, walls, and ceiling; and no drapes. And everyone was standing up, walking out to the atrium, where conversation is a little more possible.

So, how much sound can people absorb? The study Rich shared with me had the results of controlled tests of sound absorption with different numbers of people (zero, one, two, three). The results varied widely for different frequencies (more sound absorption per added person at the higher frequencies tested).

Human speech, however, was the source of the sound in our church sanctuary, and its frequencies range from an average of 125 Hz (for males) to 200 Hz (for females).

And the result? Sound absorption increased by about 5 to 20 percent (depending on the frequency) with each person added to the test chamber.

Even though I didn’t know the exact numbers of people at my church, it was a big difference between the winter months, when it’s close to full, and that summer day, with its small attendance. I estimate at least 75 fewer people. So it was not so surprising that the sanctuary was noisier the day that I, and a few others, noticed it. The bottom line? My husband was right—again. Oh, me of little faith!